The present invention relates to an apparatus for scanning a plurality of combined light beams by using a common scanning optical system and its correcting method.
There has been known a light beam scanning apparatus for scanning a plurality of light beams such as laser beams on a recording sheet put on an exposure plane by using a common scanning optical system to record an image on the recording sheet. By using a plurality of light beams, an image can be recorded at a high speed (for example, U.S. Pat. No. 5,502,709).
All of the plurality of light beams or at least light beams other than a basic or reference beam are deflected by individual optical deflecting devices independently to be controlled so that scanning lines formed by respective light beams are put in straight lines at regular intervals on a recording sheet. Since the light beams pass through a common scanning optical system simultaneously, an effect of deterioration with age of the scanning optical system is on a level for all the light beams and therefore it does not matter in general.
Prior to combining respective light beams in combining optical systems, however, the optical systems emitting the light beams are discrete and therefore deterioration with age of these optical systems directly affects positions between the light beams. Light beam intervals or phases on scanning lines fluctuate according to, for example, a temperature change or deterioration with age of the optical systems of respective light beams. It causes a problem that a quality of a recorded image is lowered.
Therefore, light beams are split by a beam splitter and the split light beams are guided to a beam position detecting device to detect respective positions of the light beams. For example, the positions of the light beams are detected, before they enter the scanning optical system, by a 4-split light beam detecting device, a PSD (position sensing device, two-dimensional position sensor) or the like which is positioned on a conjugate plane optically conjugate with an exposure plane or in a position slightly displaced from the conjugate plane. On the basis of a result of the detection, the deflections by the optical deflecting devices are corrected so that the relative positions of the light beams are appropriate.
The light beams, however, frequently show an optical power distribution which is not uniform in a circumferential direction within a range of its diameter. Accordingly it has been found that a recording position on the exposure plane after passing the scanning optical system sometimes does not precisely match the beam position before impinging on the scanning optical system detected by the beam position detecting device. FIGS. 9A. 9B are diagrams of assistance in explaining the reason why there is caused the disagreement between the recording position on the exposure plane and the beam position detected by the beam position detecting device. FIG. 9A shows an optical power distribution of the light beam, with an abscissa axis (x) indicating a position on a plane perpendicular to the light beam and an ordinate axis (p) indicating an optical power (or radiant power). As shown in this diagram the optical power (p) is unsymmetric around the maximum optical power position (x0).
On the other hand, a 4-split beam detecting device or a PSD is generally used as a beam position detecting device and they are used to detect a center of gravity of an optical power. In other words, in FIG. 9A, a position (x1) between horizontally equal areas enclosed by the distribution curve of the optical power (p) and the abscissa axis is detected as a beam position. A recording density (D) of the light beam on a film on the exposure plane is as shown in FIG. 9B. In this diagram the abscissa axis (x) indicates a position on the exposure plane and the ordinate axis (D) indicates a recording density. This recording density (D) is the highest in the position x0 where the optical power (p) is the maximum.
Therefore, unless the optical power distribution of the light beam is completely symmetric in a radial direction, an error xcex94x occurs between the beam position (x1) detected by the beam position detecting device and the recording position (x0) on the exposure plane (recording surface). This positional error xcex94x always occurs between a plurality of light beams and therefore it causes deterioration of an output image quality.
To output an image having a high resolution, it is required to expand a beam diameter of a light beam before narrowing the beam to be focused on the exposure plane. To expand the beam diameter, a lens having a large aperture or a mirror is required, while these members easily cause significant astigmatism due to a restriction on manufacturing. On the other hand, to detect the beam position of the light beam, the light beam is split before impinging on an beam expander and is introduced to the beam position detecting device. The split light beam has a small diameter and therefore astigmatism of this light beam is small. It will be described in detail below by using FIGS. 10A-10C.
FIG. 10A shows an example of a constitution of a light beam scanning apparatus of a drum inner surface scanning type. In this figure, two light beams LB1 and LB2 composed of lasers are combined with each other. These two light beams LB1, and LB2 are emitted from laser diodes LD1 and LD2 and then deflected by acousto-optic deflecting devices AOD1, and AOD2, respectively. These light beams LB1, and LB2 have sufficiently small diameters. These light beams LB1, and LB2 having small diameters are combined by a combining mirror M1and expanded in their diameters by a beam expander EX formed by lenses L1 and L2.
The expanded light beams pass through an aperture AP, guided to a condenser or focusing lens L3, and then guided to an exposure plane S by a spinner SP. The spinner SP has a mirror rotating at a high speed coaxially with the expander EX and the condenser lens L3. The exposure plane S is formed on an inner surface of the drum coaxial with the spinner SP and relatively moves in a rotary axis extending direction of the spinner SP synchronously with a rotation of the spinner SP.
In such a scanning apparatus, the light beams LB1 and LB2 having small diameters before impinging on the expander EX are split by the splitting mirror M2 and then pass through the condenser lens or image focusing lens L4 so as to be guided to the beam position detecting device PS. Therefore, a lens having a small diameter can be sufficiently used as the condenser lens L4 and its astigmatism can be also small. FIG. 10B shows an enlarged diagram of an image formation in this beam position detecting device PS. As apparent from FIG. 10B, a difference between focal positions (astigmatism) is almost zero at a view in two directions perpendicular to each other (x and y directions) on the image focusing plane. An axial z indicates a light beam traveling direction in FIG. 10B.
On the other hand, FIG. 10C shows an enlarged diagram of the image formation on the exposure plane S. As shown in FIG. 10C, there is increased a difference (astigmatism) xcex1 between a focal position on a z-y plane viewed in the x direction and a focal position on a z-x plane viewed in the y direction. This is because the aperture of the condenser lens L3 is large as mentioned above.
In this manner if there is an error xcex94x caused by an unsymmetric optical power distribution of the light beam described by referring to FIG. 9 or an error caused by astigmatism xcexc described by referring to FIG. 10, an image quality for recording is reduced. Particularly in an image setter for printing requiring a high-precision image recording, there sometimes appear moire stripes generated by a slight density change which may periodically occur on the image combined with dots. Therefore, it has a problem that an image quality may be deteriorated.
The present invention has been accomplished under the circumstances as aforementioned, and a first object thereof is to provide a correcting method for a light beam scanning apparatus capable of preventing a positional relationship of a plurality of light beams from fluctuating according to a temperature change or deterioration with age of an optical system before a scanning optical system and of preventing an image quality from being reduced by unevenness of optical power within a diameter of a light beam or by astigmatism of an optical system. It is a second object of the present invention to provide a light beam scanning apparatus directly used for an embodiment of this method.
The first object of the present invention is achieved by a method for correcting a recording position of a light beam recorded on an exposure plane by a light beam scanning apparatus, in which at least one light beam of a plurality of light beams is deflected by an optical deflecting device and said plurality of light beams including the deflected light beam are combined and scanned by a common scanning optical system to said exposure plane, comprising:
detecting respective positions of said plurality of light beams before the scanning optical system by using a beam position detecting device;
determining a first correction data for optical deflecting devices for keeping said detected relative beam positions of the respective light beams constant and storing said first correction data;
detecting the recording position of the light beam on said exposure plane to determine a deviation of the recording position from an optimum position, and obtaining a second correction data for correcting said deviation; and
adding said second correction data to said first correction data to obtain final correction data for driving said optical deflecting device.
When the scanning optical system is of a drum inner surface scanning type and one of the introduced light beams is a central beam coaxial with a rotary axis of the scanning optical system, this central beam (basic beam) may be guided to the scanning optical system bypassing the optical deflecting device. Optical deflecting devices are provided for light beams other than the basic (central) beam. Unless this central beam is used, respective optical deflecting devices may be provided for all the light beams so that all the light beams are deflected.
The first correction data (which is also referred to as xe2x80x9cbeam position correction dataxe2x80x9d, in the specification) may be stored in a first memory area. The second correction data (which is also referred to as xe2x80x9crecording position correction dataxe2x80x9d or xe2x80x9cadditional correction dataxe2x80x9d, in the specification) may include a plurality of types of recording position correction data correspondingly to resolutions on the light beam exposure plane, which are previously obtained. Such second correction data may be stored in a second memory area. With such a constitution, the second correction data corresponding to required output resolutions can be selected out of the second memory area and combined with the first correction data in the first memory area to obtain the final correction data.
A common memory of the first and second memory areas can be divided into a plurality of memory areas and different memory areas be prepared as first and second memory areas, respectively. The first and second memory areas can be configured by separate memory devices.
The second correction data (recording position correction data) can be obtained as described below. After controlling respective optical deflecting devices by using data of the beam position detecting device so as to match the respective beam positions before an introduction to the scanning optical system, it is checked that there is a difference in the recording positions of the respective light beam where an image is actually output to the exposure plane. If the difference is found, a compensation quantity is obtained for controlling a deflection quantity (angle) of an optical deflecting device to compensate this difference. This compensation quantity of the deflection quantity is considered as recording position correction data, i.e., the second correction data.
The recording position correction data can also be obtained by measuring intervals between record lines formed by a plurality of light beams onto the exposure plane and making settings so as to keep these line intervals constant. These intervals can be measured by visual observation using a magnifying lens and a measuring gauge. A plurality of types of this recording position correction data can be previously stored in the second memory area according to predetermined or desired resolutions of the image to be recorded.
The second object of the present invention is achieved by a light beam scanning apparatus for scanning a plurality of combined light beams by using a common scanning optical system, comprising:
an optical deflecting device for deflecting at least one of a plurality of light beams;
a combining optical system for combining all the light beams including the deflected light beams;
a scanning optical system for guiding the combined light beams to an exposure plane for scanning and recording an image;
a beam splitter for splitting said combined light beams;
a beam position detecting device for detecting beam positions of respective light beams in the split combined beams;
a first processing means for comparing data of a plurality of beam positions detected by the beam position detecting device with previously stored initial data and calculating a difference between both data as a first correction data;
a first memory area for storing said first correction data;
a second memory area for storing a second correction data for use in correcting relative position differences of the recording positions, the second correction data being determined on the basis of the recording positions of the respective light beams recorded on the light beam exposure plane;
a second processing means for calculating final correction data for said optical deflecting device on the basis of said first correction data and said second correction data; and
optical deflecting device control means for controlling a deflection caused by said optical deflecting device on the basis of the final correction data in synchronization with said scanning optical system.
A two-dimensional position sensor may be used for the beam position detecting device. The light beam may be a laser beam emitted from a laser diode or a solid-state laser. The scanning optical system may be of a drum inner surface scanning type. In this case, optical deflecting devices capable of two-dimensionally deflecting beams is suitable. It is possible to use, for example, two-dimensional acousto-optic deflecting devices or to combine two one-dimensional acousto-optic deflecting devices perpendicularly to each other in the above use. In addition, for the optical deflecting devices, electro-optic deflecting devices may be used instead of acousto-optic deflecting devices.
The second correction data stored in the second memory may include a plurality of types of recording position correction data, each corresponding to output resolutions and any selected recording position correction data may be used as the second correction data. Preferably, a content of the first memory area are rewritten by a rewrite instruction signal appropriately entered. A rewrite instruction for the first memory area may be sent out at power-on, once every certain elapsed time, once every image output by a certain number of pages, or a time when an operator desires.
The light beam scanning apparatus previously stores optimum relative position information of respective light beams as initialization data. First, a beam position of each light beam is detected by the beam position detecting device and then the data is obtained so as to obtain a difference from the initialization data and the difference is considered as beam position correction data. This beam position correction data is stored as first correction data in the first memory area. This correction data is stored individually in the same manner for all the light beams. In the second memory area, only correction data corresponding to output resolutions are stored as additional or second correction data, first.
If one of the plurality of light beams is considered as a basic beam and only other light beams are deflected by using the optical deflecting devices, an adjustment is required to be previously performed so that the basic beam is registered on an optical axis of the scanning optical system. In addition for other light beams, the beam position correction data is stored as described above.
Subsequently the final correction data is obtained by using the first correction data which is the content of the first memory area and the second correction data (correction data corresponding to output resolutions) which is the content of the second memory area, and light beams are scanned while correcting a deflecting quantity of the optical deflecting devices for the light beams. Specifically, the optical deflecting devices are driven so as to provide a predetermined deflection corresponding to an output resolution, by which relative beam positions of the light beams are controlled appropriately, so that the scanning line intervals correspond to output resolutions. In this manner, combined beams in which relative positions are kept appropriately are scanned by using a common scanning optical system for a scan on a recording sheet.
Scanning output lines formed with the light beams recorded on the exposure plane are observed to be checked whether there is uneven line intervals. If this unevenness is found, correction data for compensating it, in other words, data for correcting the relative position differences of the recording positions is determined and added to the data corresponding to the output resolutions so as to obtain the second correction data. The thus obtained data is stored in the second memory area. Alternatively, the second correction data is obtained by controlling respective optical deflecting devices so as to make light beams coincident on the beam position detecting device. In this case, recording position differences of the light beams on the exposure plane are observed to determine compensation quantity of the deflection of the respective optical deflecting device so as to compensate the observed difference. Such compensation quantity may be used as the second correction data.
The second processing means are used to obtain the final correction data by combining the beam position correction data (first correction data) stored in the first memory area with the recording position correction data (second correction data) stored in the second memory area. The optical deflecting device control means is used to drive the optical deflecting devices by using the final correction data. Therefore, uneven output line intervals formed with the plurality of light beams are corrected, thereby an image quality is improved.